There are several types of sights available in the market to enable a user of a weapon such as rifle, shotgun, handgun and submachine gun to aim these weapons. Examples of such sighting devises include laser sights, holographic sights, and “reflex” or “red dot” sights. Holographic sights utilize a holographic optical element (HOE), illuminated by a reconstruction beam, and the HOE reconstructs an image, typically of a reticle. A user looks through the HOE at a targeted object and perceives the reconstructed reticle. FIG. 1A schematically illustrates an example of such a prior art device. A light source 2, typically a laser diode, projects a diverging beam of light 3 which is reflected by a mirror 4 or HOE, such as a grating, creating a reflected beam 5. The reflected beam 5 in this example is also diverging, and may be considered a reconstruction beam. The reconstruction beam 5 illuminates a holographic optical element (HOE) 6 and the HOE 6 reconstructs an image of a reticle. An individual's eye 7 can view the image of the reticle and a target (not shown) through the HOE 6. This sighting device has several disadvantages. The use of diverging light to illuminate an HOE may cause drift of the image plane depth and the position of the reconstructed reticle. Also, the wavelength of light produced by typical laser diodes depends on a number of factors, including the temperature of the laser diode. For example, some laser diodes will exhibit a shift in output wavelength of approximately 0.30 nm/° C. The change in temperature of the laser diode may be due to environmental conditions or due to heating from operation of the diode. The angle of diffraction of an HOE or diffraction grating is wavelength dependent. As such, as the temperature shifts, resulting in a wavelength shift, the position of the reconstructed reticle shifts. This is undesirable. Finally, since the user views the target through the HOE, ambient light may cause a rainbow effect in high efficiency HOEs and the HOE emulsion may have defects that are detectable to the eye. Certain types of HOEs also darken over time.
FIG. 1B schematically illustrates an example of a prior art device with a configuration that compensates for wavelength shift. A light source 10, typically a laser diode, projects a diverging beam of light 11 which passes through a collimating lens 12. This creates a collimated beam of light 13. The collimated beam 13 illuminates a diffraction grating 14. The diffraction grating 14 produces a reconstruction beam 15 that is angled upwardly, in this example, which illuminates an HOE 16. The HOE reconstructs an object beam 17 that is perceived by a user's eye 18. The object beam 17 is angled downwardly, in this example, such that the object beam 17 and the collimated beam 13 are parallel to each other. The grating 14 and HOE 16 are also parallel to each other. While the grating 14 and HOE 16 still suffer from the same dispersion as in the FIG. 1A example, the dispersion of the grating is equal to and in an opposite direction to the dispersion of the HOE, thereby compensating for the wavelength shift. Such a compensation can also be achieved with a grating and HOE that are not parallel as long the dispersion of each is equal and in an opposite direction. This wavelength compensation design is sometimes referred to as an achromat or achromat configuration. This design has the same issue of viewing the target through the HOE. Also, this design can be difficult to package in a compact weapon sight, and the relative position of the various components must be maintained or the image quality or position may suffer.
There have been various attempts to provide wavelength drift compensation properties in a more compact package by folding the light path. Examples are shown in U.S. Pat. Nos. 5,151,800; 5,483,362; 6,490,060; and 7,145,703.